Interesting. A few weeks ago there was a post about mast track tuning. "Conclusion" was that for a given board one mast position can accomodate all sails. This is why *board gives recommendation for this as well. Presumably it can vary a little bit following strap settings and ride style. Another 100 Kg sailor said he puts the mast forwards whatever the sail size. In your case you went over recommended sail range but... have you tried this setting with smaller sails?

In another category (68 Kgs) I would like to use 7.3 mē on a ST104. Most people say 7 is the absolute max. Maybe would work with mast all the way forward.